Frequency modulation system



Oct. 22, 1940. .J. G. CHAFFEE FREQUENCY MODULATION SYSTEM Filed Feb. 20,1940 HA RMON/ C GENERA TOR FREQUENCY MOM/LA TCD OSCILLA TOR SIGNAL FIG/BFIG/A v IAIIVENTOR J. G. CHAFFEE A T TORNE' V Patented Oct. 22, 1940 2 tsmas -15 M115 PATENT or rice Q U v i ranooiinoyriiefiirlonsYsTniyl Q I Mh Q BellTelephone Laboratories, Incorporated, New

York, N. Y a corporation of New York Application February 20, 1940,Serial No. 319,877 i n e p 21Clai msr ((1250-47) 8 i;

The present invention relates to electricalwav e nected in thecathode-ground lead and by-passed control for signaling andkindredpurposes. More by a radio frequency condenser. For a fullerparticularly it relates to thesuppression of un disclosure of typicalembodiments the detailed p desired variations in the strength of theWaves, description to follow shouldfbe consulted. such as arise fromundesired amplitude'modula- Referring. to the drawing, v tion,distortion or noise in the system. Fig. 1 is a schematic circuit diagramof the In signaling "by means of frequency modulated invention asapplied to the harmonic generator or phase modulated waves, undesiredamplitude portion of a frequency modulated system; and

10 changesoccur from various causes either in the Figs. 1A,.1 B, and. 10show modifications that wave generating" and modulating part of the maybe made in the circuit ofFig. 1.

system or in parts of the system in which the Fig. 1 shows the inventionapplied to a harmonic transmission is not constant as to frequencygenerator in a frequency modulated wave trans-v throughout the entireband of interest. Power mitting system. Harmonic generators have beenvariations produce noise,,besides which there suggested for use infrequency modulation sys- 0 may be noisefrom extraneous sources, tubenoise, terns for the double purpose of increasing the etc, all of whichmay give rise to amplitude varig carrier frequency and increasing themodulation ations. l I v index soas to preserve a constant percentage ofIt is a general object of this invention to supfrequency shift. Thispermits generation of the press undesired amplitude variations withinfrequency modulated wave at arelatively low fre- 9 creasedeffectiveness and in a simplified manner. quency and with smallfrequency shift. The reln It has been proposed heretofore to provide aativelyhigh frequency wave actually transmitted special pick-up circuitand detector for detectand the required freq'uencyps'hiftof such high iting amplitude variations in a frequency modufrequency wave. may besecured by frequency 11 lated wave and to impress the detectedvariamultiplication in the harmonic generator. In tions on the grid ofan amplifier in the main the figure a sourceofsignal waves is. indicatedsignaling path to vary the amplifying properties at 26 connected tofrequency modulated oscilof the amplifier in such a way as to compensatelator 2| of any suitable type for producing a relfor the amplitudevariations in the waves passatively low frequency, frequencymodulatedwave ing through the amplifier. Onedifficulty enwhich isimpressed, through a tuned input cir- Q countered with this systemarises from the fact (suit, on the harmonic generator '22 comprising 30that there is a certain amount of phase shift tubes23 and 24 whoseinputs are in push-pull rearound such feedback path, and when thenumlation and whose plates are. parallel connected ber of meshesincluded in the feedback loop is to one terminal of tank circuit 25,which is couat all large, this is likelyto result in singing" at pled totransmitting antenna 26. The desired some frequency. l p harmonicfrequency is selected by proper tun- According to the present inventiona simpliing of the tank circuit 25. A by-pass condenser fied arrangementwith improvedjeffectiveness'is 29 is shown aroundthe plate source Hwhich attained by deriving the detectedamplitude variin the case of asystem using'considerable power ations in some circuitdevice in thesystem which would preferably consist of a power rectifier and 40 hasdetecting properties and using the detected filtering system as commonin the art. 40 variations to produce compensating modulation An audiofrequency impedance 9, to be dev in the same device. For example,harmonic gen-- scribed more fully presently, is connected in the eratorsor frequency changers are commonly cathode lead from the negative poleof source present in a frequency modulation transmitting H which ispreferably grounded. The grids are system and are used for shifting thefrequencies connected to the ground terminal of impedance 45 to adifferent position in the frequency spectrum. 9. A grid bias battery I0is indicated and may Such devices are efficient detectors and modubedesirable in some cases. if lators. Thepresent invention comprises theuse In the operation of the circuit o f-Fig. 1, it is of a negativefeedback coupling from the outassumed that the frequency modulated Wavesput to the input of such a device, effective to contain some undesiredamplitude modulation 50 d feed back low frequency waves havingthefrewhich is to -be eliminated. The harmonic genquency of the undesiredamplitude variations but erator 22 has a curved input-outputcharacterineffective to feed back the high frequencies. 'istic and hasthe property of producting a certain Thus, in its preferred form theinventionv may amount of amplification together with an efiicomprise anaudio frequency impedance concient degree of distortion for performingthe 55 functionof harmonic generation. As a result of the non-linearcharacteristic of this device the amplitude modulation assumed to bepresent in the frequency modulated waves is detected and appears as alow frequency detected component flowing in the anode circuit andtherefore in series through the low frequency impedance 9. This currentdevelops a voltage across the terminals of the impedance 9 which isapplied to the grid circuit in such phase as to oppose the variations ofplate current of envelope frequency, resulting in a reduction of theamplitude modulation of the high frequency waves. The action may beregarded as a negative feedback of the envelope frequencies present inthe applied wave, resulting in a reduction of the amplitude modulationappearing in the output. The action may also be looked upon ascomprising a recovery of the audio frequency modulating component in theplate circuit and remodulation of the radio frequency wave in reversesense by application of the recovered modulating component to the gridcircuit of the frequency changer.

The form which the impedance 9 of Fig. 1 may take may vary widely inpractice and illustrative examples are given in Figs. 1A, 1B, and 1C. InFig. 1A the impedance 9 is shown as comprising a resistance l3 and radiofrequency by-pass capacity [4. In this case the resistance I3 servesboth for the feedback of the detected modulating component and also forbiasing the grid. In case the grid receives an excessive negative biasan opposing battery may be used with its positive pole connected to thegrid. The manner in which the substitute network may be connected intothe circuit of Fig. 1 is shown by the dotted rectangle and the threeterminals passing through it marked respectively G, C and B.

In the form of network shown in Fig. 1B the cathode resistor I3 isreplaced by inductance Hi. This circuit would permit use of a largedegree of feedback over some desired range of frequencies where theimpedance of the combination l4, I is high, with zero or small feedbackfor both higher and lower frequencies including direct current where theimpedance respectively of condenser I 4 or inductance I5 is much lowerthan that of the combination.

In the form of the network shown in Fig. 1C a transformer consisting ofwindings l6 and I1 is used for the feedback of the detected modulatingcomponent, while avoiding the feedback of direct current voltage. Thiscircuit permits the use of a suitable voltage ratio in the transformeritself. The secondary winding l! is shown terminated in resistance [8provided with a slider to which the grid terminal is connected. Any highfrequencies appearing on the grid side of this transformer which are notto be fed back may be shunted out by condenser l9. Obviously each of thecircuits shown in Figs. 1A, 1B and 10 contains sufficient elements toenable a certain amount of shaping or control of the amplitude frequencyrelations of the fed back waves. Also, obviously, other types of cathodenetworks will appear to those skilled in the art, and the forms thathave been disclosed are to be construed as illustrative rather thanlimiting.

Instead of the one stage of harmonic generation disclosed there may be aplurality of stages in tandem for raising the frequency to the desiredfrequency level. The cathode impedance for reducing amplitude modulationmay be used in one or more of these tandem connected harmonicgenerators.

The amplitude modulations to be reduced may arise at any point in thesystem including the tubes in the frequency shifting circuits by whichthe compensating operation is carried out. For example, such undesiredmoduation may arise from tube noise, power fluctuations or other localsources, or elsewhere.

Circuits embodying the invention may be modified and varied widelywithin the scope of the claims, which follow.

What is claimed is:

1. In combination, a source of frequency modulated waves, a harmonicgenerator comprising a space discharge device having a grid, a cathodeand an anode, means to impress waves from said source on said grid andcathode, an outgoing circuit connected to Said anode and cathodecontaining filter means for selectively passing waves of desiredharmonic frequencies, and an impedance connected on one side to saidcathode and on its other side to said grid and anode, said impedanceproviding an audio frequency negative feedback path from the output tothe input of said device to oppose transmission into the outgoingcircuit of amplitude variations of audio envelope frequency in saidwaves.

2. In combination, a source of frequency modulated high frequency waves,a utilization circuit for waves whose frequencies are harmonics of thefrequencies of the waves from said source, and a spacedischargeamplifying device having an input circuit and an output circuit, andhaving a non-linear characteristic, connected between said source andsaid utilization circuit, for stepping up the frequencies to the levelof said harmonics, and means for opposing audio frequency amplitudevariations in the waves of stepped-up frequency comprising an audiofrequency negative feedback impedance common to the input and outputcircuits of said device.

JOSEPH G. CI-IAFFEE.

